rc timer circuit help

Does the 300ms start at the beginning of the 1s output duration or at the end of it? In any case, have you looked at the 74121 (various families), 74HC221, or 74HC123 to use together with an external BJT or MOSFET switch?

Jon

how precise do you need to be?

Hi

Does not need to be that precise but should be below 500ms.

You could use the first relay to dump a capacitor into the coil of the second relay. You'd have to tweak the C value a bit to get the timing right, as you'd be working against the dropout voltage of the second relay.

One cap, one resistor.

The out put is a dry contact relay.

On Thursday, July 11, 2013 9:08:02 AM UTC-4, snipped-for-privacy@gmail.com wrote: > I have a GSM controller that can switch a relay for a minimum time of 1 sec, however I need a 300ms pulse. I am looking for a circuit that can take the 1 sec relay output (as an input) and give me a 300ms output for a second relay.

If this were a system with DC power supply, and logic-level output, it's easy. If it's an AC-power (mains) system, things get trickier: the best might be an AC timed relay connected in series, that turns OFF after 300 msec, and resets when power is removed.

It matters a lot what kind of power is available (and what the nature of the intended load is).

Thanks all, going with the capacitor, resistor suggested by John, simple and effective.

Just to be clear, the resistor is in the OFF cap recharge path, to avoid contact wear. In the ON state, the cap is switched directly to the second relay coil.

Entire dial telephone systems were done with relays. Computers, too.

They also wasted a lot of power, were very slow and required a lot of maintenance. The Strowager Stepper Switchs used in telephone exchanges were motorized switches, not relays. There were relays, but the routing was done with switches.

Strowgers (no 'a') are often referred to as "relays" or "rotary relays" or "stepping relays." They work like relays and sound like relays, so I call them relays.

We could argue about "motorized" too, since there is a relay coil or two but no motor.

They are referred to a an 'Electromechanical Switch' You appear to be the only one who calls them a relay. Who ever heard of a multi-axis relay?

For anyone who's interested, here is a link to a 40 GB torrent of "Bell Systems Technical Journals".

They are also available online, one journal at a time.

Google. Lots of examples.

But call them fluglemarts if you like.

Named for a Kansas City undertaker.

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)